Substrates are often coated with a protective layer because the material of the substrate does not offer good oxidation or corrosion resistance. By contrast the coatings have good corrosion resistance but not good mechanical properties.
Both gas turbine and jet engine manufacturers must seal and isolate certain sections of an engine thereby controlling the passages of gases through a modern engine. It is well known to produce seals between sections by impressing a sharpened part feature called a seal tooth into a very crushable opposing part called a honeycomb seal.
The materials used for the honeycomb seals are selected to maintain their ability to be crushed by the sharp seal tooth. These seals are generally made of numerous pockets with six side walls similar to the pattern of honeycombs in nature. They are skived into layers about 0.2″ thick (0.6 cm) and then laid into a curved piece to fit around the engine. These pieces are called shrouds. To hold the honeycomb in, it has been the practice to braze the hexagonal shaped seal material into a shroud pocket.
To improve the life of the seals, some engine manufacturers also fill the small hexagonal pockets with abradable materials and then braze this into the shroud as well. Honeycomb seals also undergo corrosion and oxidation, which decreases the life time of the component.
These seals work acceptably well and have been in use for a number of years. As the sulfur levels have increased in fuels and harder damage has occurred to the seals during engine operation. The mechanism of damage has been shown to be hot corrosion enhanced by the sulfur levels in the fuels.
There exists a need for a more corrosion/oxidation resistant material to fabricate the cells from. FeCrAlY has been shown to be very hot corrosion/oxidation resistant but nearly impossible to fabricate. For the thin sheets which are needed to form the sidewall of the hexagonal shapes FeCrAlY is too brittle. During manufacturing it may shatter. Hence, while this material is acceptable from the corrosion/oxidation point of view it is not acceptable from difficulties during the metal fabrication.